Vehicles may include engines configured to use various operating modes in combination with a hybrid powertrain to achieve improved vehicle efficiency. One example is described in U.S. Publication No. 20050173169, which uses a homogenous charge compression ignition combustion (HCCI) engine as one form of autoignition operation. In this system, power production from the HCCI engine in operation may be decoupled from, or assist in, responding to driver power demand. When a driver power demand is greater than what can be provided by the HCCI engine, the addition of power from the motor (e.g. one or more reversible electric motor/generator(s) or reversible hydraulic pump/motor(s)) is used. Likewise, when a driver power demand is less than a value produced by the engine, a portion of power is converted into an energy form storable by an energy storage device.
The inventor herein has recognized a disadvantage with such an approach. Specifically, in some conditions, such as when the power demand is less than a value produced by the engine, the energy storage device is used to absorb the excess output produced by the engine in order to maintain the desired engine operation. However, this approach relies on the availability of sufficient energy storage capacity to absorb the excess engine output. If the energy storage capacity is insufficient, a transition to another operating mode may occur, thereby reducing engine efficiency and increasing products of combustion, under some conditions. Conversely, when the power demand is greater than a value produced by the engine, the energy storage device may be used to provide excess output in order to maintain the desired engine operation. However, this relies on the availability of a sufficient amount stored energy. If the amount of stored energy is insufficient, a transition to another operating mode may occur, again possibly reducing efficiency and/or increasing emissions.
In one approach, the above issues may be addressed by a method of operating a hybrid propulsion system for a vehicle including at least an engine having a plurality of combustion cylinders, wherein the engine is mechanically coupled to at least a drive wheel of the vehicle, an energy conversion device mechanically coupled to at least one of the engine and the vehicle drive wheel, and an energy storage device coupled to the energy conversion device, the method comprising adjusting at least one of the engine and the energy conversion device so that an amount of energy stored by the energy storage device varies within a first range when the engine is operating in a first combustion mode and varies within a second range when the engine is operating in a second combustion mode, wherein the first range is different from the second range.
Thus, it may be advantageous to adjust the operating range, such as the storage limits, of the energy storage device under some conditions to provide sufficient energy storage capacity during some operating modes (e.g. HCCI, cylinder deactivation, etc.), thereby increasing the duration of these operating modes. For example, in a hybrid electric vehicle (HEV), the upper level to where the battery is recharged may be reduced during operation in HCCI mode, thereby enabling sufficient storage capacity for absorbing energy when the requested output is less than the engine output. In another example, the operating range may be adjusted in response to the number of combustion cylinders that are deactivated. In this manner, transitions between various engine modes may be reduced, thereby enabling increased fuel efficiency and decreased levels of NOx production, under some conditions.